At one time, industrial process sites producing combustible and toxic effluent were simply vented to the atmosphere and fresh air was supplied in sufficient quantity to maintain the combustible content of the air at the site below explosive levels. As fuel prices rose and clean air laws were enacted, engineers began looking for ways to conserve energy used for heating and to clean up the exhaust air from the process site before venting it to the atmosphere.
An early step in the development of improved equipment involved the incineration of the combustible effluent and the use of heat exchanger principles to recover heat from the incinerator exhaust and to return it to the site.
A more recent development is disclosed in U.S. Pat. No. 4,255,132 issued Mar. 10, 1981, to Maximilian K. Carthew and assigned to the assignee of this application and invention. The system disclosed in U.S. Pat. No. 4,255,132 includes an incinerator which is supplied with exhaust air from an industrial process site and which functions as the primary heat source for the site. This is achieved through transfer of incinerator-produced heat to the make-up air supply. In addition, the patent teaches preheating the incinerated air and also providing a bypass around the heat exchanger to control the temperature of the system.